Power semiconductor devices which include high-voltage field-effect transistor (HVFET) devices and other integrated circuits (ICs) that dissipate high power are well known in the semiconductor arts. Such power semiconductor devices are typically encapsulated within a package that is designed for assembly onto a printed circuit board (PCB). The package usually comprises a die attach pad, typically made of an electrically conductive material such as a copper alloy, on which one or more semiconductor die are mounted. A molding compound encapsulates the semiconductor die and a portion of the leads which may extend beyond the outside of the package for electrical connection to external circuitry. The leads and die attach pad are collectively referred to as the package leadframe. Often times, the package is also designed with a mounting surface for physical attachment to a heat sink that is designed as the primary path for transfer of the heat generated by the power device away from the semiconductor die.
In certain prior art package designs the die attach pad is exposed on the mounting surface to provide low thermal impedance between the semiconductor die and the heatsink. In other package designs the die attach pad is electrically isolated from the heatsink with a thin layer of molding compound disposed between the die attach pad and the mounting surface of the semiconductor package. In these latter designs it is often desirable to maintain the thickness of the molding compound between the die attach pad and the mounting surface as thin as possible to provide a low thermal resistance between die attach pad and the mounting surface.
Many traditional power semiconductor packages with an isolated die attach pad are manufactured in leadframe strips where the die attach pad is mechanically held at only one end during the package molding process. The unsupported end of the die attach pad can therefore have significant mechanical tolerances, or move, during the injection molding of the package molding compound. This, in turn, requires an increased thickness in the molding compound between the die attach pad and the outer mounting surface of the package. An alternative manufacturing process supports the die attach pad initially and then retracts the support during the injection molding process, which improves tolerances but increases the cost of the process and therefore of the finished package. Packages in which the die attach pad is electrically isolated from the mounting surface by a thin layer of molding compound also typically requires the shortest distance along the surface of the package between the leads and the mounting surface (commonly referred to as the creepage distance) to be greater than minimum requirements.